In an image forming device such as an electrophotographic device and an electrostatic recording device, first, an electrostatic latent image formed on the photoconductor is developed with a toner. Next, as needed, a toner image thus formed is transferred onto a transfer material such as a paper sheet and then fixed thereon by various methods such as heating, pressurization or solvent fume.
In the field of such an image forming device, a digital full-color copying machine and a digital full-color printer have been put to practical use. A digital full-color copying machine produces a full-color image as follows. First, an original color image is subjected to color separation with blue, green and red filters; an electrostatic latent image corresponding to the original color image, which is composed of dots that are 20 to 70 μm in diameter, is developed with yellow, magenta, cyan and black toners; and a full-color image is formed using the subtractive color mixing effect.
Recently, there is an increasing demand for full-color images with high image quality and high resolution. Especially, to increase color reproducibility, it is hoped that an image can be printed in the same hue as ink printing.
As a color pigment for a yellow toner, for example, a disazo pigment as typified by C.I. Pigment Yellow 12, 13 and 17, and a monoazo pigment as typified by C.I. Pigment Yellow 74, 97 and 98 have been generally used.
Besides the combinations of yellow pigments, examples of improving toner properties by combining a yellow pigment and a yellow dye, are known.
Patent Literature 1 discloses a yellow toner comprising C.I. Pigment Yellow 74 and C.I. Solvent Yellow 162. In Patent Literature 1, it is described that by containing these colorants, the toner obtains satisfactory color tone that is obtained in the case of incorporating C.I. Pigment Yellow 74 solely in a toner, and high coloring power of C.I. Solvent Yellow 162.
Patent Literature 2 discloses a yellow toner comprising, as colorants, C.I. Pigment Yellow 155 and C.I. Solvent Yellow 162 at a specific ratio. Patent Literature 2 describes that due to the colorants contained at the specific ratio and due to the excellent compatibility of C.I. Solvent Yellow 162 with binder resin, the dispersibility of C.I. Pigment Yellow 155 can be further increased, and more stable charging property and better transparency can be obtained.
Patent Literature 3 discloses a yellow toner comprising C.I. Solvent Yellow 162, a condensed azo pigment (e.g., C.I. Pigment Yellow 93) and so on. Patent Literature 3 describes that the aggregation of the condensed azo pigment (e.g., C.I. Pigment Yellow 93) can be suppressed by the use of C.I. Solvent Yellow 162, so that the dispersibility of the condensed azo pigment can be increased, and more uniform triboelectric chargeability can be obtained.